Integrated circuits (ICs) have become ubiquitous. Cell phones, PDAs, cameras, medical devices, laptops, and many other devices include ICs. A typical IC includes several types of semiconductor devices, such as transistors. In modern ICs, transistors may be used to implement logic or memory functions. Typically, ICs have been planar in design. Planar semiconductor chip designs limit the amount of circuitry that may be placed on a single IC die.
To overcome some of the limitations of planar ICs, designers began stacking chips vertically to form three-dimensional designs. A three-dimensional (3D) IC, therefore, is a semiconductor assembly in which two or more planar layers of active electronic components are integrated both vertically and horizontally into a single device. These three-dimensional structures increase the density of active circuits.